1. Field of the Invention
The present invention relates to a method and a system for determining a vertical load to which a tyre mounted on a vehicle is subjected, during running of the vehicle.
2. Description of the Related Art
Inflation pressure is a convenient measurement to make on a tyre fitted on a vehicle wheel and it is becoming a standard by which tyres are monitored. Tyre load, i.e., the supported weight, is a more difficult measurement but, unlike pressure, is a direct measurement of tyre stress. Tyres are selected for a particular vehicle based on the physical strength of their structure and/or material, as well as on the normal range of vehicle weight that they should support at specified nominal temperature and pressure. If the vehicle applies a load to a tyre in excess of the load range for which the tyre has been designed, the tyre is subjected to excessive stress and may fail or have its expected lifetime shortened.
Furthermore, it has to be considered that tyre maintenance is mainly based on the duty, by the vehicle driver, of maintaining tyre pressure near a nominal value, defined by the vehicle and tyre manufacturers. Although it is well known by the tyre industry that the requisite pressure is dependent on the supported load, this load-dependent pressure information is not provided to the driver, since real-time load is unknown. As a result, should the load vary from that assumed by the manufacturer, the tyres may be improperly inflated. Since the requisite pressure increases with load, the only option left is to assume the maximum load and specify a pressure accordingly. However, this maximum pressure can: 1) give a very hard ride; 2) reduce the tyre-to-road contact area available for braking; 3) wear out the center of the tyre tread prematurely. Thus, tyre load information is needed to properly inflate tyres.
Moreover, vehicle electronic control systems, such as for example a vehicle brake control system, a traction control system, an anti-lock braking system, an electronic braking system, a steering control system, an active roll control system, a vehicle stability control system, may use information related to the tyre load, in order to control actuators that move, control and stop the vehicle.
This tyre information may be used within the vehicle, or it can be used remotely, i.e. outside the vehicle. As the telematic capability of vehicles increases, they are capable of wirelessly communicating with a remote facility for monitoring the vehicle health (diagnostics), for prediction of maintenance (prognostics), and to monitor the vehicle as it passes on the road. The information may be also historically important to understand the cause of accidents.
U.S. Pat. No. 5,793,285 discloses a method and apparatus for monitoring tyres on a vehicle, by continuously measuring the distance between the associated vehicle axle (or a vehicle body part rigidly connected thereto) and the road, while the vehicle is in operation. From this measurement, the tyre deflection is determined. According to the authors, the measured tyre deflection represents a comparatively exact measure of the respective tyre load. When the tyre deflection determined leaves a predetermined desired range, a warning signal is transmitted.
PCT patent application no. WO 03/016115 discloses a method for determining the charge or load exerted on a tyre of a motor vehicle and/or for monitoring tyre pressure, wherein the pressure in each tyre is detected during operation of the vehicle and the rotational behavior of the individual wheels is observed. Load distribution parameters are also determined by comparing the rotational behavior and/or changes in said rotational behavior of the individual wheels during given driving states, taking into account preset and/or predetermined and/or learned variables. Tyre pressure and load distribution parameters are used to determine the load or charge exerted on the tyres and/or pressure loss. In one example, a pressure-measuring system based on the use of pressure sensors (such as TPMS=Tyre Pressure Measuring System), is used to determine the tyre pressure, whilst characteristic variables representing the load distribution are determined using a system based on an evaluation of wheel speed data operating in the manner of a system (Deflation Detecting System=DDS) used to determine conditions relating to the dynamic rolling circumferences of the individual tyres. Consequently, the function of detecting capacity utilization can be set up using existing systems. In another example, the number of revolutions of a front wheel is compared with the number of revolutions of a rear wheel at the same vehicle speed or at approximately the same vehicle speed (e.g. vehicle reference speed), evaluated to obtain a load distribution characteristic variable, and the value and/or the change in the load distribution characteristic variables in defined driving situations is/are used as a means of calculating the capacity utilization or load of the tyres and/or the pressure loss.
US patent application no. 2003/0058118 discloses a vehicle and vehicle tyre monitoring system, apparatus and method for determining the load-induced deflection or deformation of a vehicle tyre. Based thereon, deflection-related information, such as tyre load, molar air content, total vehicle mass and distribution of vehicle mass, are provided. The tyre deflection region or contact region of the loaded tyre is detected by sensing the acceleration of the rotating tyre by means of an accelerometer mounted on the tyre, preferably on an inner surface such as the tread lining thereof. As the tyre rotates and the accelerometer is off of the contact region, a high centrifugal acceleration is sensed. Conversely, when the accelerometer is on the contact region and not rotating, a low acceleration is sensed. The deflection points delimiting the contact region are determined at the points where the sensed acceleration transitions between the high and low values. From a measurement of the rotation rate of the tyre, of the time between detections of the deflection points and from the tyre radius, the contact length (contactLength) can be determined. In order to determine the tyre load, the following formula is suggested:load=α×treadWidth×contactLength×pressure+forceSidewallwhere treadWidth is the width of the tread, treadWidth×contactLength is the area of applied pressure, forceSidewall is the effective resiliency of the tyre sidewall to collapse, and α is a proportionality constant near to 1. Alternatively, the load can be determined from a disclosed relationship between air moles, pressure, temperature and contact length, derived from the Ideal Gas Law.
According to the Applicant, the methods disclosed in the above U.S. Pat. No. 5,793,285 and in the above PCT patent application no. WO 03/016115 may not give reliable real-time determinations of the tyre load, since they are not based on measurements performed directly on the tyre. Thus, they may suffer from an “averaging effect”, which can cause a loss of important tyre load data, especially in rapidly varying conditions.
On the other hand, the approach disclosed in the above US patent application no. 2003/0058118 is quite theoretical and could not fit with a complex system such as a tyre. For example, considering the rectangle treadWidth×contactLength as the area of applied pressure is a strong approximation, as the contact area between the tyre tread and the road is quite different from a rectangle. Furthermore, the value forceSidewall is generally not determined with high precision, so that a further approximation would be included in the tyre load determination.